This project is a study of the classification of cells and of inhibitory interactions in the cat's dorsal lateral geniculate nucleus (LGN). The main experimental method involves recording an LGN cell on one microelectrode and using a second electrode in the eye to record simultaneously from a retinal ganglion cell that might provide input to the LGN cell. Cross-correlation techniques are used to assess whether a ganglion cell provides excitatory or inhibitory input to an LGN cell. The LGN cells studied receive input from two types of ganglion cells, X and Y. These two types may be considered parallel pathways carrying different kinds of visual information from the retina. This X and Y input is known to diverge considerably in the LGN. A major goal is to determine the extent to which this divergence represents a splitting of the X and Y input into distinct subtypes of LGN cells, each of which could constitute a parallel pathway carrying a specific kind of information to the visual cortex. I have already found two subtypes of X-cells that have distinct responses to visual stimuli and receive distinct kinds of retinal inputs. The immediate goal is to determine whether LGN cells receiving X and/or Y input divide into additional subtypes when they are characterized by their retinal inputs, and to discern differences in the visual response properties of these subtypes so that ultimately the subtype of a cell can be identified without directly recording its inputs. A description of subtypes in the LGN is important in attempting to understand and classify receptive fields in visual cortex. Studies of inhibitory interactions in the LGN involve two types of interneurons, those located in the main laminae of the LGN and those located in the perigeniculate nucleus bordering the LGN. Experiments will determine whether the intrageniculate interneurons fall into particular subtypes. Simultaneous recordings of ganglion cells and perigeniculate cells will determine the nature of the retinal input to these cells. Simultaneous recordings of perigeniculate and LGN cells will explore the interrelations between the LGN and the perigeniculate nucleus at the single-cell level. Finally, simultaneous recordings of an LGN cell and two of its retinal inputs will explore how two retinal inputs interact to produce the output of an LGN cell.